The Blue Grotto is a sea cave on the coast of the island of Capri, southern Italy. Sunlight shining through an underwater cavity is reflected back upward through the seawater below the cavern, giving the water a blue glow that illuminates the cavern. The cave extends some 50 metres into the cliff at the surface, and is about 150 metres deep, with a sandy b…The Blue Grotto is a sea cave on the coast of the island of Capri, southern Italy. Sunlight shining through an underwater cavity is reflected back upward through the seawater below the cavern, giving the water a blue glow that illuminates the cavern. The cave extends some 50 metres into the cliff at the surface, and is about 150 metres deep, with a sandy bottom.